1. Field of Invention
The present invention relates to a DC/DC converter, and, more specifically, relates to a DC/DC converter which suppresses noise generation during switching in a switched capacitor type DC/DC converter which is used for a battery driven power source circuit for a portable telephone set such as for PHS (Personal Handyphone System) and portable telephone system and a battery driven power source circuit for a portable type electronic device such as an electronic book and a PDA.
2. Background Art
In a conventional electronic device driven by a battery such as the portable telephone set for such as PHS and portable telephone system and the portable type electronic device, a circuit which is driven by a higher voltage than a normal battery voltage is incorporated. For example, an LED element drive circuit used as a back light in a liquid crystal display device and a signal transmission circuit are such examples. In order to operate these sorts of circuits a DC/DC converter for boosting the battery voltage is frequently provided as a power source circuit within these devices.
On the other hand, with regard to these sorts of battery driven electronic devices, a size reduction and light weighting of the devices themselves have been advanced, and the size of power source circuit itself has been reduced and correspondingly a circuit with a low power consumption has been demanded. In response thereto, in these sorts of devices and apparatuses, a DC/DC converter, which transfers charged electric charges to a capacitor by means of a switched capacitor such as a charge pump circuit and boosts up to an n times voltage corresponding to a so called n times voltage rectification, is provided, for example, as a part of an LED element drive circuit and a power source circuit therefor. In these sorts of devices and apparatus, the voltage boosted by such DC/DC converter is further regulated and stabilized at a constant voltage by a regulator to produce an LED drive voltage.. Thereby, a size reduction and a low power consumption of the devices is realized.
Although there are a variety of DC/DC converters of different types, however, in view of the LED element drive circuit in which voltage boosting is performed by making use of such as the charge pump circuit, it has been proposed to utilize a so called switched capacitor type DC/DC converter of which entire power source circuit is also an n times voltage rectification type.
FIG. 5 is an example of such power source circuits.
In FIG. 5, a DC/DC converter 10 includes, regardless to the LED element drive circuit, a charge pump circuit (a double voltage boosting circuit) 12 which performs switching at an oscillating frequency of an oscillation circuit (OSC) 13.
Respective terminals of a capacitor C1 of the charge pump circuit 12 are connected between an input side power source line (a positive electrode side of a lithium ion battery 11) Vin and the ground GND via respective switch circuits SW1 and SW2. Further, the respective terminals of the capacitor C1 are again connected to a charging side terminal of a power output use capacitor C2 and to the input side power source line Vin via respective switch circuits SW3 and SW4.
The charging side terminal of the capacitor C2 is connected to an output terminal Vo and the other terminal of the capacitor C2 is connected to the ground GND.
The DC/DC converter receives from the lithium ion battery 11 an electric power of a voltage of, for example, about 3.6V (usually, a certain voltage in a range of 3.0V-4.2V) and performs a boosting operation by turning ON/OFF the switch circuits SW1-SW4 in response to pulses having a predetermined frequency being outputted from the oscillation circuit (OSC) 13.
Namely, the DC/DC converter 10 turns ON the switch circuits SW1 and SW2 and OFF the switch circuits SW3 and SW4 to charge the capacitor C1 (a first capacitor), further performs a complementary switching (switching of reversing ON/OFF state of the respective switch circuits) of turning OFF the switch circuits SW1 and SW2 and ON the switch circuits SW3 and SW4 to transfer the electric charges having been charged in the capacitor C1 to the power output use capacitor. C2 (a second capacitor) after boosting substantially to doubled voltage and to charge the same.
As a result, the DC/DC converter repeats a so called double voltage rectification and generates a voltage of about 7.2V at the capacitor C2. Further, the switch circuits SW1-SW4 receive output pulses from the oscillation circuit 13 and are turned ON/OFF in response to High level thereof (hereinbelow, will be referred to as xe2x80x9cHxe2x80x9d) and Low level thereof (hereinbelow, will be referred to as xe2x80x9cLxe2x80x9d). Since the switch circuits SW3 and SW4 receive the output pulses from the oscillation circuit 13 via an inverter, the switch circuit SW3 and SW4 perform the complementary switching operation with respect to the switching circuit SW1 and SW2.
The oscillation circuit 13 performs oscillation upon receipt of electric power from the lithium ion battery 11 and outputs pulses of 50% duty ratio having a predetermined frequency to the charge pump circuit 12. Then, the DC/DC converter detects the voltage Vo at the output side which is boosted by the charge pump circuit 12 at an output voltage detection circuit 14 to feed back the same to the oscillation circuit 13 and controls the oscillation frequency of the oscillation circuit 13 so that the output voltage Vo is kept at a constant voltage.
However, in such switched capacitor type DC/DC converter, since the capacitor C1 is connected to the input side power source line Vin at the time of ON/OFF switching of the switches, noises with a high level are induced on the input side power source line Vin at the time of switching the switches. Further, in order to stabilize the output voltage Vo the oscillation frequency of the oscillation circuit 13 is controlled in which when the output voltage Vo rises, in order to lower the same the switching of the switches is performed before completing charging of the capacitor C1, therefore, noises are also induced on the output line of the output voltage Vo.
Such noises reduces electric power conversion efficiency as well as causes adverse effects to the surrounding circuits. In particular, since the frequency of the oscillation circuit varies, the frequency of the noises likely varies which makes difficult to remove the noises by a filter. Especially, such is problematic to battery driven portable type electronic devices and apparatuses.
An object of the present invention is to resolve the above problems in the conventional art and to provide a DC/DC converter which can suppress noises induced at the time of switching.
A DC/DC converter of a first aspect of the present invention which achieves the above object is characterized in that, the DC/DC converter comprises a DC power source; a reference voltage generating circuit; an amplifier which receives an electric power from the DC power source and outputs an electric power of which voltage is controlled so as to assume a target voltage value by stepping down the voltage of the electric power from the DC power source depending on a difference between the reference voltage and a detection voltage; an oscillation circuit which generates signals having a specific frequency; a voltage boosting circuit which receives the output of the amplifier and the output of the oscillation circuit, causes switching of the; output of the amplifier at the specific frequency to charge a first capacitor, and performs voltage boosting by transferring the electric charges charged in the first capacitor through complementary ON/OFF switching with respect to the former switching into a second capacitor after raising substantially upto n/m time voltage (wherein n greater than m and n and m are integers equal to or more than 2) and charging the same therewith; and an output voltage detection circuit which generates the detection voltage depending on the output voltage of the boosting circuit, whereby a voltage of substantially n/m times of the target voltage value is generated from the voltage boosting circuit.
Further, according to a DC/DC converter of a second aspect of the present invention, in place of the reference voltage generating circuit a variable voltage generating circuit is provided and in place of the output voltage detection circuit a voltage control circuit is provided which controls the variable voltage generating circuit depending on the output voltage of the voltage boosting circuit, whereby, the voltage control circuit controls the output voltage of the variable voltage generating circuit depending on the output voltage of the voltage boosting circuit, so that the output voltage of the amplifier assumes the target voltage value and causes the amplifier to generate an electric power having the target voltage value.
As has been explained in the above, according to the first aspect of the present invention, since the output voltage of the voltage boosting circuit is detected, the detected voltage is fed back to the amplifier and the output voltage of the amplifier is controlled so as to assume the target voltage value, the voltage boosting circuit which performs voltage boosting upon receipt of the output voltage of the amplifier can generate an electric power having a voltage of n/m times of the target voltage value. Thereby, the voltage boosting circuit can output an electric power having a stabilized voltage of substantially n/m times with respect to the target voltage value.
In this instance, since the voltage boosting circuit produces the boosted voltage through the switching control with the specific constant frequency, even when the boosted voltage rises above the voltage of n/m times of the target voltage value, the switching change-over during charging in the voltage boosting operation never happens to thereby suppress the noise generation. Moreover, since the switching frequency is kept constant, a circuit which facilitates noise removal in the surrounding circuits can be realized.
As a result, in the switched capacitor type DC/DC converter, the noise generation at the time of switching for voltage boosting can be suppressed.